As semiconductor devices become highly integrated, a size of a contact hole becomes small but an aspect ratio thereof becomes increased. Tungsten can be used to fill such contact holes, because the tungsten has a good step coverage. Additionally, a melting temperature of the tungsten can be as high as (or more) than 3400° C. and the tungsten has large resistances with respect to electromigration and heat.
Conventionally, a titanium layer is stacked and thermally treated to form a titanium silicide layer as an ohmic layer before filling a contact hole with tungsten (that exposes a semiconductor substrate through an interlayer dielectric layer), in order to overcome an electrical resistance difference between the semiconductor substrate and tungsten. When the tungsten is formed on the titanium silicide layer by a chemical vapor deposition (CVD) method, WF6, which is one of the source gases, may damage the titanium silicide layer. Additionally, the tungsten may have poor adhesion force with a silicon oxide that forms the interlayer dielectric layer. Thus, in order to improve the adhesion force and prevent damage of the titanium silicide layer, a barrier layer such as titanium nitride is formed before depositing the tungsten layer.
However, a titanium-containing layer such as titanium nitride has poor resistance to high temperatures (about 600° C.). If a temperature of more than 600° C. is applied in a subsequent process, the tungsten and the semiconductor substrate may react with each other to increase contact resistance. Titanium is usually formed by a chemical vapor deposition (CVD) method using TiCl4 as a source gas but this method may require temperatures of more than 650° C., which may affect a dielectric layer of a capacitor. Additionally, an electrical resistance may be increased or a short may occur due to electromigration of a metal. This electromigration may occur at an interface between different metal materials. Thus, various problems (due to electromigration) may occur between the tungsten layer and the titanium nitride layer. These problems may degrade reliability of a semiconductor device.
Additionally, when a contact plug is formed of tungsten, since source gases and process conditions for forming the tungsten are different from those for forming the titanium silicide and the titanium nitride, the process may be complicated where only one piece of processing equipment is to be used. Thus, the process may be carried out using several different types of equipment, which may complicate the process.